Question: Which of these numbers is prime? ${21,\ 41,\ 44,\ 55,\ 99}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 21 are 1, 3, 7, and 21. The factors of 41 are 1 and 41. The factors of 44 are 1, 2, 4, 11, 22, and 44. The factors of 55 are 1, 5, 11, and 55. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 41 is a prime number.